The measurement of three dimensional features on a surface has many applications. One particular example is in the inspection of electronic components and electronic component assemblies.
The most common three dimensional inspection techniques available are optical techniques. These optical inspection techniques fall into several categories, including for example, structured light imaging, interferometry imaging, and triangulation imaging. There are drawbacks with each of these categories. One of the primary drawbacks is that the techniques are highly dependent on the reflective nature of the object under inspection. For example, when an object has specular surfaces, the triangulation imaging techniques which relies on true reflections will not work properly. Furthermore, when an object is surrounded by tall components, it may cause occlusion in the triangulation imaging technique. A drawback with the interferometry imaging technique is that the measurement results are wrapped in a 2π image space. Effective measurement of objects with large steps and isolated areas is difficult with interferometry based systems.
A further category of optical inspection techniques is confocal imaging. The confocal method for three dimensional inspection does not suffer from the above mentioned problems associated with triangulation and interferometry imaging techniques.
FIG. 1 illustrates the principles of a confocal microscope. The microscope focuses light from a light source 15 onto a point at a preset focal distance from the objective lens 18. At the same time a detector 50 in the microscope detects light emitted from that same point at the same focal distance from the objective lens.
If an object is present at the preset focal distance of interest, the so-called Z distance, then the light from the light source will be incident on the object as an intense focussed spot. This intense focussed spot re-emits light by reflection and dispersion and the emitted light is detected as a high intensity peak 51 by the detector 50.
If, on the other hand, an object is located above or below the preset focal distance of interest then light from the light source will be spread over a wider circle on the object surface. Light re-emitted from the object surface, being outside the focal plane of interest, is less likely to reach the detector 50 and hence a lower intensity 51 is detected.
Confocal imaging systems only “see” structures located in the focal plane under inspection. In other words, structures located out of the focal plane are much less visible to the confocal system. Extremely good spatial (x, y) and height (z) resolution can be attained by confocal measurement. However, a drawback with the confocal method is that it is generally time consuming. Traditional confocal microscopes typically take several tens of seconds to measure per field of view. This is mainly due to the need to acquire multiple images of the field of view, each image being associated with a particular focal plane setting.
Conventional confocal microscopes use one of the following different scanning mechanisms to obtain the multiple images of a scene. These multiple images are slices or optical sections corresponding to different focal planes of the scene. Two typical scanning approaches are the spinning Nipkow disk, and axial scanning by moving the z stage. The scanning movements associated with the acquisition of multiple images are major time-consuming activities. Furthermore, in the Nipkow disk system, the disk limits the light efficiency which results in the need for very sensitive photo sensors which can considerably increase the cost of the system.
A number of publications suggest improvements to the conventional confocal microscope which are designed to enhance measurement speed. U.S. Pat. No. 5,737,084, for example, describes a means for changing the distance in the Z direction between the object and the object-position-in-focus, instead of the object stage moving in the Z direction. This means is achieved by the use of plurality of transparent flat plates between the objective lens and the object-position-in-focus in turn.
U.S. Pat. No. 5,880,465 describes a confocal microscope with low mass objective that is driven in one or more axis to improve the speed of scanning.
U.S. Pat. No. 5,847,867, assigned to Yokogawa Electric of Japan, describes a confocal microscope comprising a confocal laser scanner which rotates a Nipkow disk at high speed together with microlenses to improve light use efficiency.
These improvements are aimed at reducing the scanning time for obtaining slices of images at different focal plane. Another objective of these, improvements is to increase the light use efficiency. However, these improvements result in complex systems which are in general not fast enough for real time production measurement and inspection applications.
When imaging a surface area that is larger than the field of view of the confocal microscope, two degrees of lateral scanning movement, usually referred to as X and Y motion, are generally required. The X and Y motions are relative motions between the confocal microscope and the surface under inspection, and are typically performed by moving either the confocal microscope or the surface. In order to obtain a full three dimensional image at multiple focal planes, Z motion is also required in addition to the X and Y motions. Thus, a conventional confocal microscope needs three degrees of motion to achieve full acquisition of three dimensional data from an object or surface. These 3 motions are the focal plane scanning motion, X scanning motion, and Y scanning motion.